Device and method for reducing carbon dioxide

ABSTRACT

A device for reducing carbon dioxide includes a vessel for holding an electrolyte solution including carbon dioxide, a working electrode and a counter electrode. The working electrode contains metal hexaboride particles.

This application is a Continuation of PCT Application No. PCT/JP2011/001520 filed on Mar. 15, 2011, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a device and a method for reducing carbon dioxide.

SUMMARY

The purpose of the present disclosure is to provide a novel device and method for reducing carbon dioxide. A device for reducing carbon dioxide includes a vessel for holding an electrolyte solution including carbon dioxide, a working electrode, and a counter electrode. The working electrode contains at least one boride selected from the group consisting of strontium hexaboride (SrB₆), calcium hexaboride (CaB₆), barium hexaboride (BaB₆), lanthanum hexaboride (LaB₆), and cerium hexaboride (CeB₆). The counter electrode may contain one of platinum, gold, silver, copper, nickel and titanium.

The working electrode may contain particles of the metal boride disposed on a substrate. The substrate may be a carbon paper, a noble metal substrate, a glassy carbon substrate or a conductive silicon substrate.

The device may further include a solid electrolyte membrane interposed between the working electrode and the counter electrode. The device may further include a reference electrode.

A method for reducing carbon dioxide according to the present disclosure includes a step (a) of preparing any one of the devices as set forth above. The vessel holds an electrolyte solution containing carbon dioxide. The metal boride of the working electrode is in contact with the electrolytic solution. The method further includes a step (b) of applying a negative voltage and a positive voltage to the working electrode and the counter electrode respectively to reduce the carbon dioxide contained in the electrolytic solution. By the reduction reaction, at least one of methane, ethylene, ethan and formic acid is generated.

In the step (b), a potential difference applied between the working electrode and the counter electrode is not less than 2.0 volts.

The present disclosure provides a novel device and method for reducing carbon dioxide.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an exemplary device for reducing carbon dioxide according to the embodiment 1.

FIG. 2 shows a graph of the result of the reduction current—electric field potential measurement (C-V measurement) in the example 1, using an electrode comprising SrB₆.

FIG. 3 shows a graph of the result of the gas chromatography in the example 1.

FIG. 4 shows a graph of the result of the liquid chromatography in the example 1.

FIG. 5A shows a graph of the result of the reduction current—electric field potential measurement (C-V measurement) in the example 2, using an electrode comprising CaB₆.

FIG. 5B shows a graph of the result of the reduction current—electric field potential measurement (C-V measurement) in the example 2, using an electrode comprising BaB₆.

FIG. 5C shows a graph of the result of the reduction current—electric field potential measurement (C-V measurement) in the example 2, using an electrode comprising LaB₆.

FIG. 5D shows a graph of the result of the reduction current—electric field potential measurement (C-V measurement) in the example 2, using an electrode comprising CeB₆.

DESCRIPTION OF EMBODIMENTS

The exemplary embodiments are described below.

(Step (a))

In the step (a), a device for reducing carbon dioxide is prepared. As shown in FIG. 1, the device comprises a vessel 21, a working electrode 11, and a counter electrode 13. An electrolytic solution 15 is hold in the vessel 21. An example of the electrolytic solution 15 is a potassium hydrogen carbonate aqueous solution. The electrolytic solution 15 contains carbon dioxide. It is preferable that the electrolytic solution 15 is mild acidic in the condition where carbon dioxide is dissolved in the electrolytic solution 15.

The working electrode 11 contains metal hexaboride such as strontium hexaboride (SrB₆), calcium hexaboride (CaB₆), barium hexaboride (BaB₆), lanthanum hexaboride (LaB₆), or cerium hexaboride (CeB₆). The working electrode 11 can be fabricated as below.

First, boride particles are dispersed in an organic solvent to form slurry. Next, the slurry is applied to a porous conductive base material to obtain the working electrode 11. This base material preferably has a shape of a film. An example of the base material is a carbon paper, a noble metal substrate, a glassy carbon substrate, or a conductive silicon substrate. The working electrode may be formed by a sputtering method.

The working electrode 11 is in contact with (immersed in) the electrolytic solution 15. To be exact, the boride which the working electrode 11 comprises is in contact with the electrolytic solution 15. In FIG. 1, the working electrode 11 is immersed in the electrolytic solution 15. As long as the boride is in contact with the electrolytic solution 15, only a part of the working electrode 11 may be immersed in the electrolytic solution 15.

The counter electrode 13 contains metal. An example of the preferred metal is platinum, gold, silver, copper, nickel, and titanium. As long as the metal is not electrolyzed, the material of the metal is not limited.

The counter electrode 13 is in contact with the electrolytic solution 15. To be exact, the metal which the counter electrode 13 comprises is in contact with the electrolytic solution 15. In FIG. 1, the counter electrode 13 is immersed in the electrolytic solution 15. As long as the metal is in contact with the electrolytic solution 15, only a part of the counter electrode 13 may be immersed in the electrolytic solution 15.

As shown in FIG. 1, it is preferable that the vessel 21 comprises a tube 17. Carbon dioxide is supplied through the tube 17 to the electrolytic solution 15. One end of the tube 17 is immersed in the electrolytic solution 15.

It is preferred that a solid electrolyte membrane 16 is provided in the vessel 21. This reason is described later in the step (b). The solid electrolyte membrane 16 is interposed between the working electrode 11 and the counter electrode 13 to divide the electrolytic solution 15 into a first liquid 15L and a second liquid 15R. The counter electrode 13 is in contact with the first liquid 15L. The working electrode is in contact with the second liquid 15R.

(Step (b))

In the step (b), a negative voltage and a positive voltage are applied to the working electrode 11 and the counter electrode 13, respectively. This causes the carbon dioxide contained in the electrolytic solution 15 (to be exact, the second liquid 15R) to be reduced on the working electrode 11. As a result, at lease one of carbon monoxide, formic acid, and methane is generated on the working electrode 11. On the counter electrode 13, water is oxidized to form oxygen.

It is preferred to use a potentiostat 14 to apply a potential difference between the working electrode 11 and the counter electrode 13.

The potential difference applied between the working electrode 11 and the counter electrode 13 is preferably not less than 1.8 volts. This corresponds to the fact that carbon dioxide reduction current is measured at not more than −0.5 volts (and not less than −1.6 volts) in the example 1, which is described later.

In the preferable embodiment, the solid electrolyte membrane 16 is provided. Only a proton penetrates the solid electrolyte membrane 16. An example of the solid electrolyte membrane 16 is a Nafion (Registered Trademark) film, which is available from Dupont Kabushiki Kaisha.

The solid electrolyte membrane 16 prevents a reverse reaction on the counter electrode 13. Namely, when the carbon monoxide, formaldehyde, or methane, which is generated on the working electrode 11, reaches the counter electrode 13, it is oxidized on the counter electrode 13 to return to carbon dioxide. The solid electrolyte membrane 16 prevents this reverse reaction.

As shown in FIG. 1, it is preferred that a reference electrode 12 is further provided. The reference electrode 12 is in contact with the electrolytic solution 15. When the solid electrolyte membrane 16 is used, the reference electrode 12 is in contact with the second liquid 15R. The reference electrode 12 is electrically connected to the working electrode 11. An example of the reference electrode 12 is a silver/silver chloride electrode.

The present device and method are described in more detail by the following example.

EXAMPLE 1

Particles of strontium hexaboride (SrB6, Furuuchi Chemical, purity of 99%) having an average particle size of several microns are disposed, with a distribution density of 1×10⁷ particle/cm², on a conductive carbon paper (CP) having a thickness of 0.5 mm, thereby making an electrode catalyst (working electrode) according to the present subject matter. Using this electrode catalyst, electrochemical reduction reaction of CO₂ was performed. FIG. 1 shows a structural sectional view of the electrochemical cell used for this measurement. The electrochemical cell includes three electrodes, i.e., the boride particle supported electrode as set forth above as the working electrode 11, a silver/silver chloride electrode (Ag/AgCl electrode) as the reference electrode 12, and a platinum electrode (Pt-electrode) as the counter electrode 13. The electric potential applied to the three electrodes was changed by using potensiostat 14, and the reduction reaction of CO₂ was performed and evaluated. As the electrolyte 15, 0.1M potassium bicarbonate aqueous solution (KHCO₃ aqueous solution) was used. The working electrode 11 and the counter electrode 13 were partitioned off with a solid electrolyte membrane 16 to prevent the gases produced by the catalytic reaction from being mixed. CO₂ gas was introduced into the electrolyte 15 though the gas instruction tube 17 arranged in the vessel 21 by being bubbled in the KHCO₃ electrolytic solution 15.

First of all, (1) a nitrogen gas was introduced into an electrolyte for 30 minutes with a flow rate of 200 ml/min, keeping a bubbling state to exclude CO₂ from the electrolyte solution. Under this state, the electric potential was changed, and a curve of reduction current—electrolysis voltage (C-V curve) was measured. Next, (2) the gas was switched from nitrogen to CO₂ and the CO₂ gas was introduced into the electrolyte 15 for 30 minutes with the same flow rate of 200 ml/min so that the electrolyte 15 was saturated with CO₂. Under this state, the electric potential was changed, and a C-V curve was measured. A reduction current by CO₂ reduction reaction was evaluated by taking a difference between the C-V curve in the state (2) (the state saturated with CO₂ ) and the C-V curve in the state (1) (the state that CO₂ was excluded). FIG. 2 shows the result of the difference between the two C-V curves.

In this figure, the state that the current value (vertical axis) is negative shows that CO₂ reduction reaction occurred. As shown in FIG. 2, at the applied voltage is around −0.5 V, the reduction current changes from zero to negative in the experimental result of this example. In other words, when the electrode catalyst including the boride particles is used, the reduction current of CO₂ was observed at the voltage of approximately −0.7V with respect to the silver/silver chloride electrode (Ag/AgCl electrode) as the reference electrode. This result means that the reduction reaction has started at about −0.5V in a case using the standard hydrogen-electrode. On the other hand, when a CO₂ reducing experiment was conducted with an electrode catalyst of Cu in the same measurement system, the voltage smaller than −1.1V (i.e., larger in an absolute value) was necessary to cause the reduction reaction of CO₂. This comparison shows that the electrode catalyst which includes boride is effective in reduction of voltage for reduction reaction of CO₂.

Next, the product of the reduction reaction of CO₂ using the electrode on which the boride particles was supported was analyzed. For the analysis of gas components, a gas chromatograph of the hydrogen flame ion detector (FID) method was employed, and for the analysis of liquid components, a liquid chromatograph of the UV detection method was employed.

FIG. 3 shows the measurement result of detected methane (CH₄), ethylene (C₂H₄) and ethan (C₂H₆) with the gas chromatograph of FID. By using a separate column of PrapakQ and controlling a valve with a predetermined time-sequence, the FID gas chromatograph is programmed so that CH₄ is detected at around 1.5 minutes after the start of the measurement, C₂H₄ is detected at around 4.5 minutes, and C₂H₆ is detected at around 6.5 minutes, respectively. As a result, as shown in FIG. 3, the peaks of voltage were observed by time domains which correspond to the respective times, and it was confirmed that CH₄, C₂H₄ and C₂H₆ were generated.

The measurement result of formic acid (HCOOH) by the liquid chromatograph is shown in FIG. 4. By using a column of TSKgel SCX—H+, the liquid chromatograph was set so that a peak of HCOOH is detected at around 11.5 minutes after the start of the measurement. As a result, as shown in FIG. 4, a peak of the voltage was observed in the range correspond to this time. As a result, it was confirmed that HCOOH was generated by the reduction reaction of CO₂ by using the electrode catalyst that includes the boride particles. As set forth above, the generation of methane (CH₄), ethylene (C₂H₄), ethan (C₂H₆) and formic acid (HCOOH) were confirmed finally by the result of analysis of the product produced by the catalytic reaction.

EXAMPLE 2

As another CO₂ reduction electrode catalyst material, similar experiments were conducted with the use of calcium hexaboride (CaB₆), barium hexaboride (BaB₆), lanthanum hexaboride (LaB₆), and cerium hexaboride (CeB₆). As a result, similarly to the result obtained in with use of the strontium hexaboride (SrB₆), the generation of methane (CH₄), ethylene (C₂H₄), ethan (C₂H₆) and formic acid (HCOOH) were confirmed. In addition, similarly to the case of SrB₆, CO₂ reduction current was observed under lower voltage than that of copper.

FIGS. 5A to 5D show CO₂ reduction currents obtained with the use of the electrode catalyst supporting CaB₆ particles, BaB₆ particles, LaB₆ particles, and CeB₆ particles, respectively. The reduction current was observed at a voltage of around −0.5 volts in the case of the CaB₆ electrode and at a voltage of around −0.8 volts in the case of the BaB₆ electrode, with respect to the Ag/AgCl electrode as the reference electrode. With respect to the LaB₆ electrode catalyst, the reduction current was observed at a voltage of around −0.8 volts. With respect to CeB₆ electrode catalyst, the reduction current was observed at a voltage of around −0.6 volts.

COMPARATIVE EXAMPLE 1

For a comparison, electrolytic reaction was measured with only the carbon paper (CP) which was used to support a boron particle. As a result, an electric current by the reduction of CO₂ was not observed, and it was confirmed that CP was inactive for reducing of CO₂. The product by the electrolytic reaction was only hydrogen (H₂).

COMPARATIVE EXAMPLE 2

For another comparison, electrolytic reaction was measured with the borides of titanium (Ti) and zirconium (Zr). As a result, hydrogen (H₂) was a main product for the experiment, and hydrocarbon or formic acid (HCOOH) as a product by the electrolysis reaction was not generated.

INDUSTRIAL APPLICABILITY

The present device and method provide a novel method for reducing carbon dioxide. 

1. A method for reducing carbon dioxide by using a device for reducing carbon dioxide, the method comprising: a step (a) of preparing the device, the device comprising: a vessel; a working electrode, and a counter electrode, wherein: an electrolytic solution is hold in the vessel, the working electrode contains at least one boride selected from the group consisting of strontium hexaboride (SrB₆), calcium hexaboride (CaB₆), barium hexaboride (BaB₆), lanthanum hexaboride (LaB₆), and cerium hexaboride (CeB₆), the counter electrode contains metal, the boride is in contact with the electrolytic solution, the metal is in contact with the electrolytic solution, and the electrolytic solution contains the carbon dioxide; and a step (b) of applying a negative voltage and a positive voltage to the working electrode and the counter electrode respectively to reduce the carbon dioxide contained in the electrolytic solution.
 2. The method according to claim 1, wherein: the device further comprises a solid electrolyte membrane, and the solid electrolyte membrane is interposed between the working electrode and the counter electrode.
 3. The method according to claim 1, wherein in the step (b), a potential difference applied between the working electrode and the counter electrode is not less than 2.0 volts.
 4. The method according to claim 1, wherein in the step (b), at least one of methane, ethylene, ethan, and formic acid is generated.
 5. A device for reducing carbon dioxide, the device comprising: a vessel for holding an electrolyte solution including carbon dioxide; a working electrode, and a counter electrode, wherein: the working electrode contains at least one boride selected from the group consisting of strontium hexaboride (SrB₆), calcium hexaboride (CaB₆), barium hexaboride (BaB₆), lanthanum hexaboride (LaB₆), and cerium hexaboride (CeB₆).
 6. The device according to claim 5, wherein the counter electrode contains one of platinum, gold, silver, copper, nickel and titanium.
 7. The device according to claim 5, wherein the working electrode contains particles of the at least one boride disposed on a substrate.
 8. The device according to claim 7, wherein the substrate is a carbon paper, a noble metal substrate, a glassy carbon substrate or a conductive silicon substrate.
 9. The device according to claim 5, further comprising a solid electrolyte membrane interposed between the working electrode and the counter electrode.
 10. The device according to claim 5, further comprising a reference electrode. 